1. Field of Invention
The present invention relates to the field of seismic data acquisition systems and methods of using same. More specifically, the invention relates to methods and systems for determining one or more orientation parameters of nodes in or attached to seismic cable apparatus, such as seismic streamer and seabed seismic cables.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. For large survey areas, a seismic spread may have vessels towing multiple seismic streamer cables through the water, and one or more seismic sources by the same or different vessel. The seismic sources typically comprise compressed air guns for generating acoustic pulses in the water. The energy from these pulses propagates downwardly into the geological formations and is reflected upwardly from the interfaces between subsurface geological formations. The reflected energy is sensed with seismic instruments attached to or internal of the seismic streamer cables, and data representing such energy is recorded and processed to provide information about the underlying geological features. So-called “ghost” signals, which reflect off of the surface of the water, may be problematic. Typically, it is necessary to “de-ghost” seismic signals received by seismic instruments. Seismic data may also be acquired using one or more so-called seabed seismic cables, and on land using a variety of seismic techniques. While this discussion focuses on art related to towed streamer seismic data collection, the invention is not so limited.
The orientation of seismic receivers is critical for the purpose of de-ghosting using multiple seismic instruments. Ghost signals may be separated from the directly reflected seismic signal if the ghost signal is recorded by two or more seismic instruments in seismic streamer cables with known fixed separation. Unfortunately, due to seismic streamer cable rotation, snaking and other irregularities, the distance between any two seismic instruments in a streamer are almost constantly changing. Accelerometers may be used for measuring streamer cable rotation, but accelerometers are subject to drift, add weight to the streamer, and are therefore not ideal. An alternate method of measuring orientation of seismic cable apparatus would be beneficial to the de-ghosting effort. The methods and systems of the present invention address this problem, as well as providing other advantages in certain embodiments.